doc: add missing require to example in http2.md

PR-URL: https://github.com/nodejs/node/pull/21858
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/http2.md

@@ -1921,6 +1921,7 @@ instances.
 
 ```js
 const http2 = require('http2');
+const fs = require('fs');
 
 const options = {
   key: fs.readFileSync('server-key.pem'),